This application claims Paris Convention priority of DE 102 05 625.0 filed Feb. 12, 2002 the complete disclosure of which is hereby incorporated by reference.
The invention concerns an NMR (nuclear magnetic resonance) resonator with at least one RF (radio frequency) resonator for emitting and/or receiving RF signals at one or more desired resonance frequencies to and/or from a measuring sample in an investigational volume, disposed about a coordinate origin (x,y,z=0), of an NMR apparatus with a means for producing a homogeneous magnetic field B0 in the direction of a z axis, wherein normally conducting conductor structures of the RF resonator, which act inductively and partially also capacitively, are disposed between z=xe2x88x92|z1| and z=+|z2| substantially on a surface which is translation-invariant (=z-invariant) in the z direction at a radial (x,y) separation from the measuring sample.
An arrangement of this type is known from DE 34 14 559 [2].
The present invention concerns the field of high-resolution nuclear magnetic resonance (NMR), in particular the configuration of normally conducting resonators for receiving the NMR signal from the NMR measuring sample.
One of the main problems of normally conducting resonators is their magnetic susceptibility, i.e. the diamagnetic or paramagnetic properties of the conductor, which can strongly deteriorate the homogeneity of the static magnetic field in the measuring volume and therefore the resolution of the NMR spectrum. To prevent this, the conductors are generally configured from several material components with different diamagnetic and paramagnetic properties and the mass proportion in percent of the individual material components is selected such that the overall susceptibility of the conductor is exactly zero, if possible.
Despite this measure, a residual susceptibility usually remains which is produced through the existing tolerances for zero compensation of the overall susceptibility. A copper conductor is an example thereof. It is highly diamagnetic and its susceptibility value can be compensated for to approximately 1% of the copper value by adding paramagnetic material portions. Such precise compensation is however difficult to achieve during manufacture and generally increases the rejection rate. For this reason, it is desirable to find methods which produce satisfactory results even for large compensation errors.
It is the underlying purpose of the present invention to present a new type of normally conducting NMR resonators having additional conductor structures which are optimally decoupled from the actual RF resonator to optimally compensate for the disturbing influence produced by the susceptibility of the conductor.
This object is achieved in accordance with the invention in an NMR resonator having the above-mentioned features in that an additional compensation arrangement is provided on the surface which is translation-invariant in the z direction (=z-invariant), which extends to values of at least z less than xe2x88x92|z1|xe2x88x920.5|r| and z greater than +|z2|+0.5|r|, wherein |r| is the minimum separation between the measuring sample and the compensation arrangement, with the compensation arrangement comprising further conductor structures which are largely RF-decoupled from the RF resonator, wherein the conductor structures of the compensation arrangement and of the RF resonator are composed of individual surface sections (xe2x80x9cZ-structuresxe2x80x9d) which comprise conductor structures and which are disposed in the z-invariant surface to each extend across the entire length in the z direction of the conductor structures of the compensation arrangement and of the RF resonator and whose conductor structures are disposed such that, with suitable conceptual decomposition of the areas of the Z structures into a plurality of small, equally sized surface elements which differ only with respect to their z position, a largely identical mass of normally conducting material would be present in all of the surface elements.
In the inventive resonator, the individual normally conducting conductor portions which carry the radio frequency (RF) current and which therefore form the RF resonator are supplemented by additional normally conducting conductor portions which are disposed quasi continuously within the RF resonator, which extend beyond same in the z direction and which are, to the extent possible, RF-decoupled from the RF resonator such that they do not carry RF current. FIG. 9b shows an arrangement built according to this principle, wherein the RF resonator is shown with hatched lines and the additional conductor parts, which merely serve to homogenize the B0 field in the active measuring region, are shown in black. Clearly, the conducting material is distributed fairly homogeneously within each one of the three vertical structure surfaces which are oriented parallel to the z axis to effect a uniform distribution of the dipole moments caused by the susceptibility of the conductor material. This produces a nearly vanishing disturbing field in the measuring volume such that the NMR spectrum is no longer significantly influenced.
The terms NMR resonator, RF receiver coil arrangement, and RF resonator will be mentioned several times in the following description. Their meanings are similar and they are therefore defined now to clearly distinguish them from another.
An NMR resonator represents the entire resonator arrangement. It is composed of one or more, preferably 2 or 4 RF receiver coil arrangements which are disposed around the measuring volume and which can be RF-coupled to one another. The RF receiver coil arrangement itself comprises the RF resonator and the compensation arrangement, wherein the RF resonator substantially represents that part of the RF receiver coil arrangement which carries the RF current.
In order to construct and analyze the inventive RF receiver coil arrangements, it is useful to conceptually divide their overall conductor structure into stripe surfaces parallel to the z axis, with each surface being formed from a single row of identical, small surface elements. To produce effective compensation of the susceptibility effect, identical amounts of magnetic dipole moment must be provided within each surface element of an individual strip, i.e. identical amounts of conductor material.
The smaller the elements of the conductor structure, the finer possible division of the total surface of the RF receiver coil arrangement into strips with identical, small surface elements. The smallest dimension of the surface elements must not be less than the smallest dimension of the structural elements since individual surface elements could otherwise fail to contain any conductor material at all thereby violating the condition of identical dipole moments per surface element. As fine a surface division as possible is required to minimize the waviness of the disturbing field in the active measuring region produced by the magnetic dipole moments of the conductor material in the individual surface elements. Reasonably fine division can be obtained when the total number of the surface elements is larger than 50, preferably larger than 200.
The most important aspect of this divisioning is the number of identical surface elements which differ only with regard to their z position, i.e. disposed on strips oriented parallel to the z axis. This number should be larger than 20 and preferably larger than 50.
In one particularly preferred embodiment of the inventive RF receiver coil arrangement, the conductor structures of the compensation arrangement project past both sides of the RF resonator by at least half, preferably approximately twice, the extension of the RF resonator in the z direction. The edge regions of the compensation arrangement which are mainly responsible for the disturbing influences in the active measuring region are thereby spatially moved as far from the active measuring region to preclude significant influence at that location.
NMR resonators are usually composed of several RF receiver coil arrangements in order to optimize the filling factor. In an advantageous embodiment of the invention, the NMR resonator therefore contains several coupled RF receiver coil arrangements, preferably 2 or 4 on different partial regions of the z-invariant surface.
In one embodiment of the invention which is particularly simple with respect to geometry and production, the conductor structures of the RF resonator(s), which act inductively and partially also capacitively, and the conductor structures of the compensation arrangement(s) are both disposed on planar substrate elements which are oriented parallel to each other and to the z axis.
One obtains a particularly good filling factor when the inductively and partially also capacitively acting conductor structures of the RF resonator(s) and the conductor structures of the compensation arrangement(s) are disposed on cylindrical substrate elements which are concentric to each other and which are arranged parallel to the z axis.
One embodiment of the inventive RF receiver coil arrangement is particularly simple to produce wherein the inductively and partially also capacitively acting conductor structures of the RF resonator and the conductor structures of the associated compensation arrangement are disposed in the same plane or on the same cylindrical surface.
An alternative embodiment is characterized in that the conductor structures of the RF resonator and the associated compensation arrangement are disposed on two or more flat or cylindrical partial surfaces which are parallel or concentric to one another, wherein the first partial surface comprises the inductively and partially also capacitively acting conductor structures of the RF resonator and parts of the associated conductor structures of the compensation arrangement and the remaining parts of the conductor structures of the compensation arrangement are disposed in the other partial surfaces. This somewhat complicated arrangement produces a further physical degree of freedom which permits very exact compensation of the disturbing influence produced by the susceptibility of the conductor structures.
In a further development of this embodiment, the separations between the partial surfaces, measured in a direction perpendicular to the partial surfaces are not more than 600 xcexcm, preferably not more than between 50 and 200 xcexcm. These separations must be kept as small as possible to minimize errors in the compensation of the susceptibility influence.
One simple embodiment of the invention is particularly preferred wherein the conductor structures of the compensation arrangement and of the RF resonator are disposed on the same partial surface of the z-invariant surface. A plurality of further embodiments, which are characterized by their simple construction, can be derived from this embodiment.
In a further particularly preferred very simple embodiment of the invention, at least part of the conductor structure sections is disposed like strips which are either parallel or perpendicular to the z axis. This embodiment has a simple geometrical shape and serves as a starting point for the production of different z structures.
In one embodiment which increases variation options, the conductor structures on the z-invariant surfaces have different geometric shapes, e.g. strips of different orientation and width and squares, circles, trapezoids etc. This increases the flexibility for design of the overall geometry to further optimize error compensation.
A very useful Z structure which can be used individually or multiply in an inventive RF receiver coil arrangement consists of identical, narrow conductors which are disposed at close intervals, periodically and continuously in the z direction, wherein the separation between neighboring conductors is small compared to the smallest separation |r| from the investigational volume.
A further very useful Z structure which can be applied individually or multiply in an inventive RF receiver coil arrangement, consists of individual conductors which are oriented parallel to the z axis and have identical or different mutual separations.
In one embodiment of the inventive RF receiver coil arrangement, the conductor structures which do not belong to the RF resonator contain a number of narrow interruptions which are distributed over parts or over the entire length of the individual conductor sections. This considerably improves RF decoupling between the RF resonator and the compensation arrangement since the currents which are usually produced in the conductor structures are minimized or suppressed by the interruptions.
In a further embodiment of the invention, the RF resonator is produced from the desired regions of the Z structures in that the conductor or parts thereof is/are rotated or displaced within small surface elements of the Z structures, and/or narrow separations between the conductors are galvanically connected and/or narrow interruptions are fashioned in the conductors and/or inclined Z structures are used and/or the Z structures are distributed over two or more partial surfaces.
A very concrete, particularly preferred embodiment which has a relatively simple geometrical structure is characterized in that the RF receiver coil arrangement is formed of three Z structures, two with vertical strips on the left and on the right and one with horizontal straight strips in the middle.
In another embodiment of the invention which provides very exact compensation and many variation possibilities, the entire RF receiver coil arrangement is disposed in two planes and is composed of three Z structures, i.e. of a first and second structure with strips arranged parallel to the z axis at a defined mutual separation and a third structure with strips disposed periodically in z and transverse to the z axis, wherein the third Z structure is positioned above the first two structures and precisely covers these, wherein the RF resonator and the compensation structure are generated in that the three Z structures are partially decomposed and distributed on two separate partial surfaces which belong to one or two substrates, wherein the first partial surface contains the entire RF resonator with the main part of the compensation structure and the second partial surface contains the remaining part of the compensation structure, and the structures produced in this fashion are positioned on top of each other on the two partial surfaces such that the sum of their structures is once more equal to the sum of the original three Z structures, thereby producing an operative NMR resonator.
Further advantages can be extracted from the drawings and the description. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail by means of embodiments.